In the case of measuring the thickness of a coating by using ultrasonic waves, a method is known in which ultrasonic waves are transmitted to an object having a coating formed on the surface and the thickness is determined from the difference in the propagation times or sound velocities of reflected waves between the top and bottom surfaces of the coating. However, the above method cannot be utilized when the reflected waves of sufficient strength cannot be had from the boundary surfaces because there is only a small change in an acoustic characteristic between the coating and the base metal or when the difference of the propagation times of reflected waves cannot be made distinct because the coating layer is too thin.
In contrast, when the attenuation of ultrasonic waves between the coating layer and the base metal differs, the following methods are available. First, there is a method disclosed in JP-A-6-18487. In this method, with regard to a cylindrical object having a hardened layer on the outer part, ultrasonic waves are transmitted to the cylindrical object in the chord direction, transmitted ultrasonic waves in the chord direction of said object are received, a frequency spectrum of the transmitted ultrasonic waves at each position of the chord is obtained by scanning the position of the chord in the diameter direction, and the depth of said hardened layer is determined based on the change in these spectral frequency patterns. Second, according to "Acoustic Properties of Plasma-sprayed Coatings and their Applications to Non-destructive Evaluation," Thin Solid Films (Vol. 83, No. 3, Pages 311-324, 1981), a coating thickness is determined by measuring the attenuation of ultrasonic waves transmitted through plasma-sprayed coatings.
In JP-A-6-18487 referred to above, since a change in the received waves is observed by changing the position of a chord, which is measured by scanning the position of a probe that transmits and receives ultrasonic waves, and by propagating ultrasonic waves, which go from a transmitter to a receiver, through a coating layer under different conditions, the use of this method is limited to a case for a chord direction of a cylindrical material. In contrast, since the method of the Thin Solid Films reference referred to above obtains beforehand a relationship between amplitudes of transmitted waves and coating layer thicknesses and converts amplitude into thickness based on this relationship, it would be sufficient to obtain ultrasonic waves transmitted through the coating layer; and there would be little limitations on the object to be measured. However, when there are changes in the amplitudes of transmitted waves by causes other than due to thickness and when the amount of contribution by these causes is unknown, the use of this method will not suitable. The reasons can be found in the following cases. First, there is a case of an ultrasound transmit/receive apparatus not directly contacting the object to be measured. In this instance, ultrasonic waves are transmitted to the object via a medium used to establish contact with the object (called a contact medium), but the acoustic characteristic in this case induces change in the transmission rate. Second, there is a case of an ultrasound transmit/receive apparatus being placed only on one side of the object. In this instance, ultrasonic waves are reflected at the boundary between the reflection surface of the object and the contact medium, but the reflection rate is changed because of the acoustic characteristic in this case. In both of these cases, there are changes in the apparent attenuation because of the changes in the energy distribution at the boundary surface between the object and the contact medium. This apparent attenuation is different from the attenuation that depends on the thickness of the coating layer of the object. If the acoustic characteristic of the contact medium is known, these two different attenuations can be separated, but if not known, they cannot be distinguished, and consequently, a large estimation error can result because of the changes in the apparent attenuation.